(Mountain Lion’s feature, known as Lion Recovery prior to Mountain Lion’s release, is a big help here, but not all Macs get it—and if your Mac’s drive is itself having trouble, recovery mode may not even be available. Also, if you need to reinstall Mountain Lion, recovery mode requires you to download the entire 4GB installer again.) Finally, if you need to —assuming you have the license to do so—a bootable install drive makes it easier to do so.
So if you buy a new Mac post-Mountain Lion, and you haven’t purchased Mountain Lion for another Mac, you can’t download the Mountain Lion installer from the Mac App Store. Once Apple starts shipping Macs with a Mountain Lion version of , I’ll update that article to cover Mountain Lion. There’s a file inside the installer that indicates which version of OS X it will install, but getting to that file and viewing it is messy. The easiest approach is to simply look at the Information box on the on the Mac App Store—specifically, check the date next to Updated. [Editor's note: This article is part of our .] Like Lion (OS X 10.7) before it, Mountain Lion (OS X 10.8) doesn’t ship on a disc—it’s available only as an installer app downloadable from the Mac App Store, and that installer doesn’t require a bootable installation disc. But there are a good number of reasons you might want a bootable Mountain Lion installer on an external hard drive or a thumb drive (USB stick).